The individual forms of cytochrome P-450 display unique substrate specificity and reactivity profiles toward a variety of drugs and carcinogens. Differences in cytochrome P-450 phenotype may relate to individual differences in sensitivity to certain drugs and susceptibility to chemical carcinogenesis. Monoclonal antibodies (MAbs) to various forms of rat liver cytochrome P-450 have been used as specific probes for the cytochrome P-450s in human liver. Western blot analysis with MAb to rat ethanol-induced P-450, a form with high nitrosamine metabolizing activity, detected a related P- 450 in human liver microsomes. Individual variation in the level of this P-450 was observed. A radioimmunoassay (RIA) to this P- 450 was developed with MAb 1-98-1. Using this assay we were able to detect and quantitate the MAb-specific cytochrome P-450 in human liver microsomes. This human P-450 was immunopurified and analyzed by peptide maps and amino acid sequencing.